In these years, due to the high concern about the environmental issues, various biodegradable aliphatic polyesters typified by polylactic acid are attracting attention.
Polylactic acid is a resin relatively high in melting point, but the glass transition temperature (Tg) thereof is low. Therefore, the injection molding of polylactic acid are unfortunately limited in the operation conditions such as the die temperature and the molding cycle time, for the purpose of controlling the crystallinity of polylactic acid. Therefore, the development of the heat resistance of polylactic acid involves technical constraints. Additionally, the impact resistance and the durability (hydrolysis resistance) of polylactic acid are far from being sufficient.
The biodegradable aliphatic polyesters other than polylactic acid, such as polyethylene succinate, polybutylene succinate and polyglycolic acid are also far from being sufficient in heat resistance, impact resistance and durability.
As described above, the biodegradable aliphatic polyesters are limited in physical properties and productivity when used as molding materials. Hence, it has come to be considered that the biodegradable aliphatic polyesters can hardly be used each alone.
On the other hand, the following viewpoint has come to be widespread: even the improvement of the performances of biodegradable aliphatic polyesters by mixing non-biodegradable resins therewith is favorable to the environment if the biodegradable resins originate from plants just like polylactic acid, because the wide use of such biodegradable resins reduces the use of the resins originating from petroleum and consequently contributes to the saving of the petroleum resources.
For the purpose of compensating such aforementioned deficient performances of the biodegradable aliphatic polyester resins, alloys of such polyester resins with polycarbonate resins that are excellent in heat resistance and impact resistance have been proposed.
For example, JP-7-109413A discloses a resin composition including a polylactic acid resin and an aromatic polycarbonate resin. The resin composition is improved with respect to the heat resistance and the impact resistance to a practical level as compared to polylactic acid alone. However, the material obtained from the resin composition has pearlescence and hence does not provide uniform exterior appearance, and hence unfortunately the scope of use thereof is limited.
JP-2005-048067A discloses that in a resin composition including a polycarbonate resin and a polylactic acid, the impact resistance thereof is improved and the pearlescence thereof is diminished by mixing an impact improver such as a core-shell graft polymer in the resin composition. However, the improvement effect of the exterior appearance attained herewith is not sufficiently satisfactory.